This invention relates to the recovery of heavy oil from subterranean sand formations containing viscous tar-like petroleum materials. In one of its aspects this invention relates to the recovery of heavy oil by thermal driving techniques. In still another of its aspects this invention relates to maintaining optimum porosity of the tar sand formation during thermal driving of heavy oil through the formation. In still another of its aspects this invention relates to the introduction of foam into a tar sand formation as an aid in thermal drive technique for removing heavy oil from a tar sand reservoir.
Large deposits of tar sands such as those in the Athabasca district of Alberta, Canada, have been found in various parts of the world. These tar sand deposits can extend over large areas and in some places form reservoirs over 200 feet thick. These reservoirs sometimes occur close to the surface of the earth but, more generally, are covered by considerable overburden. These more deeply buried reservoirs constitute a considerable amount of the world's presently known petroleum deposits with the sand usually containing from about 6 percent to about 20 percent by weight of heavy oil. It is with reservoirs penetrated by wells at a depth in the range of 1200 feet or more that this invention is particularly concerned.
The heavy oil present is sufficiently viscous at the usual reservoir temperature, about 50.degree. to 90.degree. F, that it is immobile. If the temperature is raised above about 200.degree. F the viscosity is decreased sufficiently that heavy oil will become mobile. The mobility of the heavy oil usually causes the sand formation to become incompetent along fractures produced in the usual techniques for recovering heavy oil from the reservoirs. This incompetency of the reservoir structure causes the heavy oil-depleted sand to fall away from the fracture walls by gravity and can lead to plugging of the porous portions of the reservoir or can lead to channeling of the steam used to heat the reservoir so that the steam passes through the reservoir without raising the temperature of the heavy oil the formation adequately to produce mobility of the heavy oil so that enough of it is produced from the formation to make the economics of the operation commercially attractive.
Thermal-drive methods, typically employing an injection well and a production well penetrating a reservoir formation with a hot fluid introduced through the injection well to transfer heat from the hot fluid as it passes through the formation to the production well so that the viscosity of the heavy oil within the formation is lowered and the heavy oil can be driven to the production well, are most commonly used to recover petroleum tar from underground tar sands. The relatively impermeable nature of the tar sand reservoirs requires that a fracture must be extended between the injection and production wells prior to or during the injection of the hot drive fluid. Even with fracturing techniques it has been found necessary to maintain pressure within the fractures to sustain the porosity of the fractures. This pressure is generally described as enough to overcome the pressure of the overburden weighing down upon the fracture.
If pressure is not maintained within the fracture it will tend to close and revert to its relatively impermeable state. The tar sand formations are usually sufficiently incompetent that propping agents do not prevent the closing of the fractures.
Maintaining sufficient pressure within the fractures to assure continuation of the porosity of the formation can cause the heating fluid to break through into the producing well at excessively high rates compared to the heavy oil being produced. This channeling of the heating fluid makes the operation uneconomical. A method for creating backpressure within the fractures and preventing, or at least retarding, excessive flow of steam through the fractures can be of great economic importance in the recovery of heavy oil from tar sand deposits.
It is therefore an object of this invention to provide a method for building a backpressure within the fractures of a tar sand deposit with the heated fluid used in treating the deposits. It is also an object of this invention to provide a method for retarding the flow of heated fluid through the fractures in the tar sand deposit being treated. It is another object of this invention to provide an improved method for recovering heavy oil from tar sand deposits.
Other aspects, objects and the various advantages of this invention will be apparent upon reading this specification and the appended claims.